Effects of Poly(acrylic acid) Electrosteric Stabilizer on Entry and Exit in Emulsion Polymerization

Abstract

A methodology developed to obtain rate coefficients for entry and exit (desorption) in emulsion polymerizations was applied to systems stabilized electrosterically by a copolymer of acrylic acid and styrene embedded in a styrene seed particle. This was grown as a second-stage procedure, by adding styrene and acrylic acid to a styrene seed and then polymerizing. Rate coefficients for entry (ρ) and exit (k) for subsequent homopolymerization of the resulting latices with styrene were obtained from the time dependence of the approach to steady state using both chemical and γ-radiolytic initiation; the latter was used in relaxation mode, which measures k directly. Compared to the same latices with an electrostatic stabilizer, at pH 7 the electrosteric stabilizer greatly reduced both ρ and k. When ionic strength was increased, ρ increased relative to that found for electrosterically stabilized latex in the absence of added electrolyte. For electrostatically-stabilized latices, entry is supposed to occur by aqueous-phase propagation to a critical degree of polymerization z which then undergoes irreversible entry; the present data for electrostatically-stabilized latices support this model, including its prediction that ρ be independent of particle size, all other things being equal. The decrease in ρ in the electrosterically-stabilized latices is ascribed to a “hairy” layer through which diffusion of a z-mer is slow, so that it may be terminated prior to actual entry. For electrostatically-stabilized latices, exit is supposed to occur by transfer, resulting in a monomeric radical which exits by diffusing through the aqueous phase, this event competing with intraparticle propagation; the decrease in k in the electrosterically-stabilized latices (also seen in other polymerically-stabilized systems) can be interpreted by assuming that aqueous-phase diffusion is slower in the hairy layer.

Seeded emulsion polymerization of styrene: influence of acrylic acid on the particle growth process

“…The systems we studied, however, are different because we used a polystyrene seed latex containing no carboxylic groups. Carboxylated seed latexes were also used by Coen et al 30 to study the influence of pH on the emulsion polymerization of S. Poly(acrylic acid) as stabilizer played a crucial role in the mechanism of the polymerization, which is probably also the case in the work of Shoaf and Poehlein. 9,10 Our systems actually describe the carboxylation process of polystyrene latexes and in this context the limited effect of pH on the polymerization rate is plausible.…”

Sterically Stabilized Emulsion Polymerization of Styrene

“…36 The data in Table I, however, differ from those reported for the electrosterically stabilized emulsion polymerization of styrene where the desorption of radicals was depressed. [8][9][10] The decreased desorption of radicals was attributed to the thick emulsifier layer formed by the graft copolymer at the polymer particle surface. This difference between the present data and the literature ones might result from the difference in the solubility, the molecular weight and mobility of the stabilizers used.…”

“…This may not be a case for the high molecular weight copolymer. [8][9][10] The reduction in the polymerization rate with increasing the weight ratio of emulsifier and monomer can arise from the breakdown in the extent of compartmentalization of the free radicals in the latex particles and the increased exit of radicals. The decreased extent of compartmentalization of radicals is accompanied with the shapes of the conversion and polymerization rate curves (Figures 1 and 2) typical for the microemulsion polymerization characterized by the strong radical desorption.…”

“…For example, the electrosterically-stabilized latexes show somewhat smaller values of both entry (k a ) and exit (k des ) rate coefficients as compared to the electrostatically-stabilized latex particles of the same size. [8][9][10] The decreased k a or k des in the sterically-stabilized latexes is ascribed to a ''hairy'' layer around the particles, which retards diffusion of oligomeric radicals. The entering radicals may be terminated prior to the actual entry into the latex particles occurs.…”

Target Synthesis of Functional Biocompatible Nanocomposites with “Core-Shell” Structure

“…They also suggested that flocculation of seeded and overcoated particles, poorly stabilized due to swelling in monomer at the absence of surfactants. Effect of stabilizing effectiveness of styrene and acrylic acid copolymer on proceeding of elemental stages of water dispersed polymerization of styrene is studied [38]. It is shown [38,39] that stabilizer forms highly viscous zones around the particles and it leads to decreasing diffusion rates of monomer and oligomer radicals inside PMP and restriction desorption of radicals from particles surface.…”